I. Field of the Invention
This invention relates to modified polyurethane adhesive compositions having improved viscosity and bond strength characteristics. This invention also relates to a process for laminating substrates using the polyurethane compositions herein.
II. Description of the Prior Art
Polyurethane adhesives, because of their excellent adhesive properties, are particularly desirable for use in bonding substrates. The most widely used adhesives for preparing flexible packaging laminations are based on curable polyether and polyester urethanes of low molecular weight, which are supplied as flammable solvent-borne systems of two types: one-part and two-part, both of which cure by reaction of isocyanate (NCO) groups. Other solvent-borne adhesives are fully reacted polyurethanes of high molecular weight. Water-based urethane adhesives are also known in the art.
As an alternative to these adhesives, the 100%-reactive, solvent-free adhesive system may be used, which is of two distinct types: (a) the reactive liquid adhesive, which is applied at room temperature and cured by radiation or through chemical combination, and (b) the reactive hot-melt adhesive, a solid at room temperature which, when heated to moderate temperatures (82.degree.-121.degree. C.), melts to a highly viscous liquid and cures upon contact with ambient moisture.
The reactive liquid urethane adhesives of type (a) may be composed of one or two parts. Unsatisfactory initial bond strength and short pot life after mixing are two disadvantages of the two-part adhesive. The reactive one-part adhesive is the most convenient of the reactive liquid adhesives, yet because of the high viscosity of such adhesives, a solvent is normally added thereto prior to application. In addition, residual monomers which remain after curing radiation-curable reactive liquid adhesives may produce odors or undesirable extractables.
There are several hot-melt urethane adhesives of type (b) known in the art. These urethane-based systems are superior to conventional hot-melt adhesives, which latter adhesives depend on their high molecular weight for their cohesive properties and hence have high viscosities of from 5000 to 10,000 cps. at temperatures of 150.degree.-190.degree. C. Furthermore, the conventional hot-melt compositions generally contain a variety of ingredients and are plagued by stability problems because of the high temperatures involved in their use. Finally, because these compositions are not curing, they are thermoplastic and lack resistance to solvents and heat.
The typical hot-melt urethane adhesives of the prior art contain a polyurethane prepolymer of low viscosity (such as is exemplified in Belgian Pat. No. 835,022), to which are added tackifiers, copolymers, thermoplastic polymers, adhesion promoters, etc., in varying mixtures, to enhance the initial bond strength and overall performance of the prepolymer (see, in particular, U.S. Pat. Nos. 3,931,077 and 3,991,025). Usually a combination of such additives is necessary to achieve sufficient bond strength in adhesives of this type, resulting in complicated formulations.
It is an object of the present invention to provide a polyurethane adhesive of simple and convenient composition having improved viscosity characteristics as compared with adhesives of the prior art.
It is another object to provide a modified adhesive composition having good bond strength, and, in one embodiment, offering good resistance to heat.
It is a further object to provide a process for laminating substrates and to provide a laminated film assembly wherein the modified urethane composition described herein is employed as the adhesive.